1. Field of the Invention
This invention relates to drum brakes and, in particular, to a variable position anchor assembly that permits adjustment of the position of brake shoes relative to the brake drum to account for wear.
2. Discussion of Related Art
A conventional drum brake includes a brake drum that rotates with a wheel or wheels proximate to one end of an axle. The brake drum defines a radially inner braking surface. A brake spider is disposed about the axle and supports an anchor pin that extends from the spider. A pair of brake shoes are pivotally mounted at one end on the anchor. The opposite end of each brake shoe is engaged by an actuator such as a cam or hydraulic piston or wedge to move the brake shoes between positions of engagement and disengagement with the braking surface of the brake drum.
Over time the clearance between the brake shoes and the braking surface of the drum increases with wear of the brake shoe linings and drum. The clearance may also increase due to expansion of the drum due to heating from the braking action. The increase in clearance leads to a degradation in braking performance and ultimately requires costly and time consuming maintenance operations to replace the linings. Brakes commonly include mechanically activated automatic slack adjusters to reposition the brake shoes periodically and restore the clearance as wear occurs. Conventional slack adjusters, however, have several drawbacks. First, conventional slack adjusters are located at one end of a long camshaft controlling the brake actuator and therefore are somewhat removed from the actual surfaces that are being adjusted (i.e., the surfaces of the brake lining and the drum). Flexure and sometimes poor fit between the various components located between the slack adjuster and the lining/drum surfaces can result in non-optimum adjustment (either under adjustment or over adjustment). Second, conventional slack adjusters are typically “one way” in that they are only designed to reduce the clearance between the brake linings and drum and do not “back up” if the lining swells or other factors cause too tight of a fit between the lining and drum surfaces. For example, while a vehicle descends along a long grade, the brakes heat up causing expansion of the drum. Conventional adjusters compensate for the expanding drum. When the vehicle reaches the bottom of the grade and the brakes cool, however, the drum—and the gap between the drum and brake linings—shrinks which can result in a gap that is smaller than desired and can even result in undesirable contact between the braking linings and drum (a dragging brake).
The inventors herein have recognized a need for an anchor assembly for a brake that will minimize and/or eliminate one or more of the above-identified deficiencies.